Recently, a semiconductor memory utilizing a variable resistance element as a memory element, for example, a magnetic random access memory (MRAM) has attracted attention and has been developed. The MRAM uses, as a memory element, a magnetic tunnel junction (MTJ) element utilizing a magnetoresistive effect such that the resistance varies according to the magnetization direction. Particularly, a large resistance variation can be attained in a tunneling magnetoresistive (TMR) element utilizing a TMR effect.
The MTJ element has a structure in which one nonmagnetic layer (for example, an insulating layer) is sandwiched between two ferromagnetic layers. The magnetization direction of one (fixed layer) of the ferromagnetic layers is fixed, but the magnetization direction of the other ferromagnetic layer (recording layer) is not fixed and the magnetization direction of the recording layer is set in parallel to or anti-parallel to the magnetization direction of the fixed layer. The TMR effect is a phenomenon in which the resistance of the MTJ element varies according to the relative relationship (parallel/anti-parallel) between the two magnetization directions. Specifically, when the magnetization direction of the recording layer is set in parallel to that of the fixed layer, a current tends to pass through the insulating layer (the resistance thereof becomes smaller) and when the magnetization direction of the recording layer is set in anti-parallel to that of the fixed layer, a current is difficult to pass through the insulating layer (the resistance thereof becomes larger). If the relative relationship between the two magnetization directions is set to correspond to “0” or “1”, a memory element by means of which data written in the MTJ element can be read according to the resistance can be formed.
In the above MRAM, a so-called spin-transfer MRAM with which data write and read operations are performed by directly passing a current through the MTJ element is known. When reading data in the spin-transfer MRAM, it is necessary to prevent occurrence of an erroneous write operation caused by a read current. For this purpose, for example, the direction of the read current is set equal to and in parallel to the magnetization directions of the recording layer and fixed layer (for example, Jpn. Pat. Appln. KOKAI Publication No. 2007-115320).
However, the process of determining the direction of the current is based on a bi-directional critical current in which inversion of magnetization occurs at the write time is symmetrical (the absolute values are equal). In practice, however, the critical current in magnetization inversion obtained by experiments is asymmetrical and has a problem that the asymmetric property is not taken into consideration. Therefore, there occurs a possibility that erroneous writing (write disturbance) occurs when reading data.